Hello,
I have a tube phonostage, the PAS Z-PH10 from Erhard Audio, as part of a complete PAS Next Gen kit from him. The phonostage output sounds muffled, like the high frequencies are very suppressed. I have reached out to Erhard Audio and he mentioned he did not have any suggestions for me, never encountered this problem before. I tested every single resistor before I installed them to make sure they were the correct values before installation. I have reflowed every solder joint in case there was a cold solder joint. I am using brand new 12AY7 tubes from Electro Harmonix and I tried swapping their positions with no change. Erhard Audio told me the RIAA section of the unit is R4, C3, R8 and C6. I did try replacing C6 already because the original cap was specified as a 5% piece and I happened to have 1% caps of identical value on hand, but that also made no difference. I have plenty of resistors on hand if I need to swap any parts out, and I have spare caps if I need to swap them out. Also, not sure if it makes enough of a difference, but the voltage out of the top regulator section is 220v instead of the specified 240v. Voltage in to regulator is 267v. I get the impression Erhard audio was not concerned about the difference. Could just be difference caused by wall voltage of course.
I am confident the problem lies in the phonostage as I can use my solid state phonostage on the Aux input and the preamp section sounds just fine. The problem is only when using the tube phonostage. Not just dark, but very muffled like when your ears are plugged up. It's also odd that the problem is in both channels, so whatever mistake I made or whatever bad part was used...well, I must've done it twice lol.
Attached is a PDF of the phonostage schematic.
Any suggestions?
I have a tube phonostage, the PAS Z-PH10 from Erhard Audio, as part of a complete PAS Next Gen kit from him. The phonostage output sounds muffled, like the high frequencies are very suppressed. I have reached out to Erhard Audio and he mentioned he did not have any suggestions for me, never encountered this problem before. I tested every single resistor before I installed them to make sure they were the correct values before installation. I have reflowed every solder joint in case there was a cold solder joint. I am using brand new 12AY7 tubes from Electro Harmonix and I tried swapping their positions with no change. Erhard Audio told me the RIAA section of the unit is R4, C3, R8 and C6. I did try replacing C6 already because the original cap was specified as a 5% piece and I happened to have 1% caps of identical value on hand, but that also made no difference. I have plenty of resistors on hand if I need to swap any parts out, and I have spare caps if I need to swap them out. Also, not sure if it makes enough of a difference, but the voltage out of the top regulator section is 220v instead of the specified 240v. Voltage in to regulator is 267v. I get the impression Erhard audio was not concerned about the difference. Could just be difference caused by wall voltage of course.
I am confident the problem lies in the phonostage as I can use my solid state phonostage on the Aux input and the preamp section sounds just fine. The problem is only when using the tube phonostage. Not just dark, but very muffled like when your ears are plugged up. It's also odd that the problem is in both channels, so whatever mistake I made or whatever bad part was used...well, I must've done it twice lol.
Attached is a PDF of the phonostage schematic.
Any suggestions?
It looks to me like C4 and the resistors around it are supposed to realize the 75 us time constant, but that that depends a lot on what is connected to the output. With open output, the time constant is much too large. With a long cable and a high input resistance load, it is even worse. With a short cable and precisely the right load resistance, it could work. I think this is the main issue.
Another thing is the input capacitance. It is probably quite small, which is fine for some cartridges, but not for others.
Edit: Tildo was faster.
Another thing is the input capacitance. It is probably quite small, which is fine for some cartridges, but not for others.
Edit: Tildo was faster.
Yes, not your fault, that's a terrible output filter circuit. It is designed for only one value of load,
and will be all over the map with the normal variations in loading of different preamps.
A different RIAA topology, all between the two stages, would eliminate this unbuffered mess.
Like this: https://www.kabusa.com/riaa.htm
Otherwise, redesign it for your specific line stage preamp's input resistance (which may be the volume control).
If you are lucky, only adjusting R12 will be enough to set the 75uS pole correctly, when using your particular value
of loading. But use short, low capacitance cables to the line stage (they are part of the load too).
and will be all over the map with the normal variations in loading of different preamps.
A different RIAA topology, all between the two stages, would eliminate this unbuffered mess.
Like this: https://www.kabusa.com/riaa.htm
Otherwise, redesign it for your specific line stage preamp's input resistance (which may be the volume control).
If you are lucky, only adjusting R12 will be enough to set the 75uS pole correctly, when using your particular value
of loading. But use short, low capacitance cables to the line stage (they are part of the load too).
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In response to Rayma and MarcelvdG, this is part of a kit designed to work together, so I would think it is perfectly configured to be used with it's mate, the Aikido Classic Line Preamp. I will post the schematic tomorrow so we can verify. Meanwhile, I will check C4 and the surrounding resistors, as well as C5 just because and post back what I find. Maybe I will also check the first resistor in the preamp circuit since it appears to be quite essential. I do not know enough about the circuit design to have checked that far. Thanks for the replies so far!
I checked and tested all of the relevant resistors and they all tested fine as well in the right spots. They are 1% resistors and measure as such. I did not get a chance to remove and check C4 yet. Other than it is labeled correctly as 2000pf 250v cap. Attached is the schematic for the preamp section. My volume and balance controls are after the line stage. The output of the phonostage goes into a 6 position selector switch from Glassware, which connects directly to the line stage. I am using teflon coated 22ga sliver plated copper wire for all signal wire.
I roughly calculated the correct value for C4 and found 1.003104968 nF, so please try 1 nF for C4.
C4 realizes the second RIAA correction pole, so you definitely need it, its value is just wrong for the present load resistance.
By the way, R13 is based on an incorrect rumour about Neumann record cutting equipment and can best be shorted, but its effect is very small anyway.
C4 realizes the second RIAA correction pole, so you definitely need it, its value is just wrong for the present load resistance.
By the way, R13 is based on an incorrect rumour about Neumann record cutting equipment and can best be shorted, but its effect is very small anyway.
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Regarding the calculation:
Without the two 1 Mohm resistors and without output load, the open-loop gain of the line amplifier would be close to -50 and the impedance at the virtual ground node would be about 20 kohm. I guesstimate 30 kohm with two times 1 Mohm and output load.
Add the 220 kohm and the input resistance of the line stage is 250 kohm.
In parallel to that, you get 1 Mohm (R15) and the output resistance of V2 including the effects of R11, R12 and R14.
The internal resistance of V2 including the effect of the feedback via R14 is mu * (R14 + 1/gm). According to r-type.org mu is 44 and gm about 1.75 mS for a 12AY7; of course that depends to some extent on the bias point, but I just took these values.
((mu * (R14 + 1/gm) in parallel with R11) plus R12) in parallel with 1 Mohm and with 250 kohm is 74.76784822 kohm. I've neglected R13, which shouldn't be there anyway. 1 nF will then get you close to 75 us time constant.
Without the two 1 Mohm resistors and without output load, the open-loop gain of the line amplifier would be close to -50 and the impedance at the virtual ground node would be about 20 kohm. I guesstimate 30 kohm with two times 1 Mohm and output load.
Add the 220 kohm and the input resistance of the line stage is 250 kohm.
In parallel to that, you get 1 Mohm (R15) and the output resistance of V2 including the effects of R11, R12 and R14.
The internal resistance of V2 including the effect of the feedback via R14 is mu * (R14 + 1/gm). According to r-type.org mu is 44 and gm about 1.75 mS for a 12AY7; of course that depends to some extent on the bias point, but I just took these values.
((mu * (R14 + 1/gm) in parallel with R11) plus R12) in parallel with 1 Mohm and with 250 kohm is 74.76784822 kohm. I've neglected R13, which shouldn't be there anyway. 1 nF will then get you close to 75 us time constant.
Please don't change anything until asked to.
The RIAA must be redesigned for the exact line stage that you will use with it,
which has a 220k input impedance.
If I use the KAB link mentioned above, given an R1 of 137k, R2 and C1 are correct values in Erhard's circuit, but C2 is before the second gain stage while in the PH10 circuit C2 is after the gain stage. The KAB calculator suggests C2 should be .005 uF. Given the above conversation, it sounds logical C2 would now be a different value since it is in a different position. Technically the calculator suggests 20k for R2 and this circuit has 21k there.
Erhard said to me initially he has never heard of anyone complaining about any muffledness from this circuit and doesn't know why it would do that. Perhaps I just have some bad C4 caps?
It would not be hard at all for me to remove R13 and C4, and place a 1uF cap to connect the traces appropriately as a test. I have a really nice Hakko desoldering gun so I can just as easily and cleanly put R13 and C4 back if it doesn't work.
Yes, I recall reading as well that Broskie's Aikido preamp design does not use global NFB. It was added to this modified version. Might've been related to getting the circuit to fit into this compact form factor, but don't hold me to that lol. I tried looking for the forum page where I read that but haven't found it yet.
Erhard said to me initially he has never heard of anyone complaining about any muffledness from this circuit and doesn't know why it would do that. Perhaps I just have some bad C4 caps?
It would not be hard at all for me to remove R13 and C4, and place a 1uF cap to connect the traces appropriately as a test. I have a really nice Hakko desoldering gun so I can just as easily and cleanly put R13 and C4 back if it doesn't work.
Yes, I recall reading as well that Broskie's Aikido preamp design does not use global NFB. It was added to this modified version. Might've been related to getting the circuit to fit into this compact form factor, but don't hold me to that lol. I tried looking for the forum page where I read that but haven't found it yet.